1. Field of the Invention
The invention pertains to guidance systems for tools and machines, more specifically to a tool holder carriage and guidance system which travels over a predetermined course at a predetermined speed over a round, flat, or variable geometry surface precisely and repeatedly.
The term "tool holder carriage" is defined as means for mounting an item, machine or tool on a guide track of the invention for moving along the track, and includes a portion of an item, machine or tool that is adapted to mount on the guide track for moving along the track.
This invention is useful for guiding semi automatic and fully automatic pipe and tube welding tools, and for guiding other tools and machines over the surface of a workpiece.
2. Description of the Prior Art
An essential feature of most orbital welders is a compact welding module containing motors and mechanisms to achieve the various required motions such as arc gap control, oscillation, dwell, wire feed, orbital welding positioning, programming switching, and propulsion.
Accurate placement and speed of propulsion over the workpiece is mandatory to the physics of the welding arc, most commonly in the TIG or MIG processes. Requisite speed control is 0.5% to 3% of programmed speed.
It has been traditional in the prior art to employ a propulsion technique for the tool holder or welder holder that is based upon a device which is dependent upon closely toleranced pitch length such as the pitch length of a gear pitch or roller chain. This pitch-based design works for pipes of selected diameter but carries a heavy penalty of ancillary economic limitations. For example, the pitch length of a gear rack propulsion track must relate to the diameter of the geared drive wheel which rides the track in an exact multiple of the unit pitch of the propulsion unit so that the teeth of the gear and teeth on the track mesh. If the necessary matching length of the track is not compatible with the diameter circumference of the workpiece, then adjustable structures to bridge the gap between the track and workpiece must be provided.
The gear teeth are transverse to the direction of propulsion and do not prevent lateral slippage between the tool holder drive couple of wheel and rack. A separate guide track and idler follower are added to control lateral slippage, adding cost to the arrangement.
The adjustable features are further complicated if the diameter of the workpiece involves many critical tolerances. The necessity for adjustable structures raises the possibility that the annulus between the track and the workpiece may go out of concentricity and complicate the weldment. The need to provide a stiff structure to stabilize the chain pitches or the gear rack closely toleranced tooth geometry, and a circle of precisely tolerance diameter, has traditionally demanded an expensive lath turning, with the raw material typically starting as a forged billet or a forged ring. Since quantities of forgings are usually limited in this highly specialized industry, specially ordered forgings or castings have been economically inappropriate. In large sizes, 12 inch diameter or larger, the product cost rises to an unacceptable level, and in very large sizes, 24 inch diameter or larger, the availability of open time on large lathes becomes an obstacle.
Welding tool holders having gear trains between the electric motor armature and the propulsion reaction point, and with backlash at the chain or gear tooth propulsion element, accumulate large total back-lash, particularly since the motor gear box frequently has five or more stages of gears. The total back lash unloads as the welding tool holder moving around a pipe traverses from a climbing torque against gravity to a reverse braking torque with gravity. The change in torque from positive to negative is accompanied by an uncontrolled momentary change in speed and displacement. This is a critical anomaly in the physics of a welding arc, causing poor quality weld easily quantified by a welding monitor or identified by the naked eye. This sudden torque reversal and the subsequent change of travel speed is herein called "lurch". The term includes a mechanical or electrical backlash occasioned by passing from positive (propulsive) to negative (braking) phases of the machine or the reverse.
U.S. Pat. No. 3,456,555 patented Jul. 22, 1969 by L. E. Dunlap describes a guide for cutting tools having a flexible rubber guide track, triangular in cross section, attached to the top surface of the workpiece. A tool carriage has a first idler wheel and a second idler wheel on one side of the tool carriage. The second wheel is spaced apart and tandem on the guide track from the first wheel. A third idler wheel configured to roll directly on the top surface of the workpiece is mounted on the other side of the carriage lateral to the first and second wheels. Each of the first and second wheels has a circumferential groove in which the side walls of the groove fit the angled side walls of the top of the track. The tool is mounted on the carriage between the first and the third wheel.
U.S. Pat. No. 3,476,161 patented Nov. 4, 1969 by L. E. Dunlap describes a guide for a saw blade. A plate is mounted horizontally on the top surface of the work piece. A first idler wheel having a circumferential groove rests on a track mounted on the plate adjacent to a first side of the saw blade. The saw blade extends below an edge of the plate for cutting the work piece as the first wheel is moved along the track. The walls of the groove contact the track laterally to the length of the track. A drive motor for the saw extends laterally from the second side of the saw blade. An outrigger arm extends from the motor, laterally past the first idler wheel to a bracket that is mounted on the plate and is spaced from the first idler wheel. A second idler wheel that is mounted on the end of the idler arm distal from the first idler wheel bears upwardly against a downward facing surface of the bracket, the downward facing surface being spaced upward from the plate. The first idler wheel is forced down against the track by force of gravity by the weight of the motor wherein the track is the fulcrum pivot between the motor spaced from one side of the track and the bracket spaced from the other side of the track.
U.S. Pat. No. 3,259,021 patented Jul. 5, 1996 by J. S. Appleton et al. describes a carriage mounted on a rigid, elongate, flat ribbon track by first and second rollers each having an annular, parallel wall groove rolling on one edge of the track. The first and second rollers are held against the track by a third roller having an annular, parallel wall groove on the opposite edge of the track between the first and second rollers in staggered three point contact with the track. The first and second rollers are mounted on a first truck that is attached to the carriage by a pivot between the first and second rollers that is parallel on its axis to their axis. In another arrangement the first and second rollers on the pivoted truck are opposed by third and fourth rollers mounted on a second truck that is mounted on the carriage by a pivot between the third and fourth rollers that is parallel on its axis to their axis. The first and second rollers are drawn to the rigid ribbon track by force applied across the track through the pivots.